1. Field of the Invention
The present invention relates to an image printing apparatus for printing an image on a printing medium such as a plain paper and a plastic sheet by using an ink or a developing agent.
2. Description of the Related Art
As a conventional apparatuses for printing an image on a printing medium such as a paper, an ink-jet printing apparatus and a thermal transfer printing apparatus are known.
In an ink-jet printing method, ink drops are ejected from a small nozzle, and pixels are selectively formed on a printing medium by using the ink drops, thereby obtaining a desired image. This method is economically advantageous because pixels can be formed by ejecting an ink to only required portions. However, since a nozzle having a small diameter corresponding to the size of an ink image to be formed is used, clogging of the nozzle often occurs due to drying of an ink in the nozzle, precipitation of impurities therein, and the like. Especially in a multi-nozzle type head having a plurality of nozzles arranged in correspondence with the width of a printing sheet, it is difficult to solve this problem of clogging. Therefore, it is difficult to manufacture such a head for practical use.
In a thermal transfer printing method, an ink ribbon obtained by coating a hot-melt or a heat-sublimable ink on a support member composed of a polyester film or the like is used, and thermal energy is selectively applied to the ink ribbon by means of a thermal printing head or the like, thereby transferring the ink onto a printing medium. The thermal transfer printing method is advantageous in that an image having a relatively high contrast and excellent half-tone characteristics can be obtained. However, this ink ribbon cannot be repeatedly used, and the residual ink which is not transferred, and the support member on which the ink is coated are discarded after a printing operation. Therefore, this method is not economical.
With the recent widespread use of multi-media, video information tends to be easily processed in homes. Along with this tendency, the demand has arisen for a compact domestic image printer for obtaining a hard copy based on image information..
Meanwhile, a thermal transfer printer using a heat-sublimable ink is most popular as a compact domestic image printer. According to the sublimation type printer, a dye-ink ribbon is pressed against a printing sheet, and a thermal printing head behind the ink ribbon is heated to cause sublimation or thermal diffusion of the dye contained in the ink ribbon, thereby forming an image on the printing sheet. This system is capable of density modulation of about 64 to 256 tone levels per dot and hence is suitable for high-definition, multi-image printing, but is high cost with respect to recording paper.
The need for an electrophotographic printer represented by a laser beam printer increases on the market because it ensures high speed and high image quality. The principle of this laser beam printer will be briefly described below.
In the laser beam printer, the surface of a drum-like photoreceptor is charged to -600 V by a charger (precharge). A laser beam emitted from a laser diode is then deflected by a polygon mirror to one-dimensionally scan the charged photoreceptor. Exposure is imagewise performed by ON/OFF-controlling the output of the laser beam. Note that an actual apparatus includes a plurality of optical elements between the laser diode and the photoreceptor. Portions, of the charged photoreceptor, on which the laser beam is irradiated lose charges, and their potentials decrease to voltages near 0 V, thereby forming an electrostatic latent image. A bias voltage of -500 V is applied to a negatively charged toner to bring the toner near the photoreceptor on which the latent image is formed. Toner particles are attracted to only the portions on which the laser beam has been irradiated. As a result, a toner image is formed. The photoreceptor on which the toner image is formed is pressed against a printing sheet, and charges are applied from the rear surface side of the printing sheet by the charger, thus transferring the toner image to the printing sheet. The toner image on the printing sheet is heated by a heat roller to be fixed. With this operation, printing is completed. Meanwhile, the photoreceptor which has passed the transfer section is uniformly discharged by a lamp, and the residual toner on the photoreceptor is cleaned by a blade or the like, thus restoring the initial state.
Although such an electrophotographic printer is advantageous in terms of operation speed and the like, a complicated printing process is required. Especially a laser beam printer requires a space for scanning a laser beam and hence tends to increase in size. In addition, with this increase in size, the weight of the printer tends to increase.